(518a) Accelerated Predictions of Small-Molecule Diffusion in Polymers

Molecular diffusion in polymeric medical device materials underlies a wide variety of public health issues related to unintended leaching from or uptake into implantable medical devices. However, accurate diffusion coefficients for these systems at physiological temperature are challenging to obtain, both experimentally and computationally. Although molecular dynamics (MD) simulation can accurately predict the diffusion coefficients (D) of low-molecular-weight gases in polymers, it has proven difficult to extend this success to larger molecules, such as condensable vapors, liquids, and drugs. These heavier molecules diffuse much more slowly, which typically requires extremely long MD simulations to reach converged estimates of D [1, 2]. Here, we discuss alternatives that may enable more rapid assessments. We examine the suitability of various accelerated sampling techniques (e.g., parallel replica dynamics [3]) for extending the effective timescale of molecular diffusion in soft matter, which presents some complications not encountered with the typical application to atomic diffusion in solids. Moreover, we describe a link between D and dynamic localization, characterized by the Debye-Waller factor (u²). Over a wide range of temperatures above the glass transition, there is a linear relation between ln D and 1/u². This suggests a route to faster predictions because u² can be quantified in very short MD simulations [2].

1. C. Forrey, D. M. Saylor, J. S. Silverstein, J. F. Douglas, E. M. Davis, Y. A. Elabd. Soft Matter 10, 7480-7494 (2014).
2. D. M. Saylor, S. Jawahery, J. S. Silverstein, C. Forrey. J. Chem. Phys. 145, 031106 (2016).
3. A. F. Voter, F. Montalenti, T. C. Germann. Annu. Rev. Mater. Res. 32, 321-346 (2002).